This research will be done primarily in Russia as an extension of NIH grant # R0l GM45744. Dosage compensation is a striking example of the interplay between gene-specific regulation and chromosomal architecture. This process has evolved to make X-linked gene expression equivalent in males with one X chromosome and females with two. In species examined at the molecular level, dosage compensation is mediated by sex-specific factors that decorate the X chromosomes to regulate chromatin structure and gene expression. In Drosophila, dosage compensation is achieved, at least in part, through site-specific histone H4 acetylation, modulated by a male-specific, X-specific complex (composed of the MSL proteins, and non-coding roX RNAs). Our focus in this FIRCA proposal, enabled by the cytological expertise of the Zhimulev group, will be to analyze the exquisite X chromosome-specificity of the Drosophila dosage compensation complex. We have recently proposed that in wild type males, the MSL complex forms at approximately 30 chromatin entry sites, distributed exclusively along the X, and is then attracted in cis to sequences or proteins that may be common to active genes throughout the genome. This remarkable property of spreading in cis will be examined by mapping the sites to which the complex spreads in the endogenous X chromosome, as well as in cases where a transgene carrying a chromatin entry site is inserted ectopically into an autosome. We will also study conditions that affect the ability and site-specificity of MSL spreading, including heat shock, limiting levels of the MSL2 subunit, and the behavior of X:A translocation chromosomes. In both flies and humans, regulatory molecules are normally restricted in cis to the X chromosome, but if brought to autosomes, can spread on genes never before dosage compensated. Dissecting the mechanisms underlying these epigenetic regulatory processes will provide insight into many important biological problems, including normal and disease states in humans. The superb spatial resolution of polytene chromosomes, a defined initiation site for spreading, and the availability of mutants in the protein and RNA spreading components make the MSL complex an ideal model system to determine how changes in chromatin architecture affect gene expression in complex organisms.